In recent years, as portable devices such as portable telephones and notebook computers have spread, the development of a compact and lightweight secondary battery having a high energy density is desired. As this kind of secondary battery is a lithium-ion secondary battery that uses lithium, lithium alloy, metal oxide or carbon as the anode, and research and development of such a secondary battery is being actively performed.
A lithium-ion secondary battery that uses a lithium composite oxide, and particularly, a lithium cobalt composite oxide capable of being relatively easily synthesized for the cathode material is able to obtain high 4V class voltage, so is expected to become a battery having a high energy density, and practical application of such a battery is advancing. In regards to batteries that use lithium cobalt composite oxide, much development has been performed in order to obtain excellent initial capacity characteristics and cycling characteristics, and various results have been obtained.
However, lithium cobalt composite oxide uses a cobalt compound which is rare and expensive as a raw material, and so causes the cost of the cathode material and lithium-ion secondary battery to increase. A lithium-ion secondary battery that uses lithium cobalt composite oxide as the cathode material has a unit cost per capacity that is approximately four times that of a nickel-metal hydride battery, so uses and applications are quite limited. Therefore, lowering the cost of the cathode material, and making it possible to produce a less expensive lithium-ion secondary battery has large industrial significance from the aspect of promoting more lightweight and compact portable devices.
As a cathode active material that can alternate for the lithium cobalt composite oxide is a lithium nickel composite oxide that uses nickel, which is less expensive than cobalt. Compared to a lithium cobalt composite oxide, a lithium nickel composite oxide displays a lower electrochemical potential, and decomposition due to oxidation of the electrolyte does not easily pose a problem, so achieving a high capacity is expected. Moreover, as in the case of a lithium cobalt composite oxide, lithium nickel composite oxide displays a high battery voltage. For these reasons, currently, much research is being actively carried out for a lithium nickel composite oxide that can be applied as a cathode active material for a lithium-ion secondary battery.
In order to obtain the excellent battery characteristics, or in other words, high output, low resistance, high cycling and high capacity of the lithium-ion secondary battery, it is necessary for the cathode active material to be composed of particles that have a uniform particle size, and that come in suitable contact with the electrolyte.
For example, in order to achieve battery characteristics such as high output and low resistance, sufficient contact between the cathode active material and the electrolyte must be maintained. When the contact between the cathode active material and the electrolyte is not sufficient, the reaction surface area cannot be sufficiently maintained, and the reaction resistance increases, so a high-output battery cannot be obtained. Moreover, the cycling characteristic is related to the particle size distribution, and using a cathode active material having a wide particle size distribution causes the voltage that is applied to the particles inside the electrode to become uneven, and when charging and discharging are repeated, fine particles selectively deteriorate and the capacity is decreased. Furthermore, the battery capacity is closely related to the filling ability (packing density) of the cathode active material. More specifically, when the packing density is high, it is possible to fill a larger amount of cathode active material inside an electrode having the same volume, so it is possible to increase the battery capacity. As a method for increasing the packing density, increasing the particle size when the particle density is the same is effective. However, mixing in particles having too large of a particle size will cause the filter to clog when performing filtering after mixing the cathode paste, and will cause thin and long shaped defective parts to occur during coating.
Here, the lithium nickel composite oxide used as the cathode active material is obtained by calcining a nickel composite hydroxide precursor together with a lithium compound. The particle characteristics such as the particle size and particle size distribution of the cathode active material are basically inherited from the particle characteristics of the nickel composite hydroxide precursor. Therefore, in order to obtain a desired particle size and particle size distribution of the cathode active material, it is necessary to obtain such a particle size and particle size distribution in the precursor stage.
The reactive crystallization method is typically used as a method for producing a nickel composite hydroxide precursor. In order to obtain particles having a sharp particle size distribution, a method of performing crystallization in a batch operation is effective, however, the batch method has a disadvantage in that productivity is worse than in a continuous method that uses an overflow technique. Furthermore, in order to obtain large particles in a batch operation, it is necessary to increase the amount of raw material that is supplied, and as a result, the reaction vessel must be larger, thus productivity becomes even worse.
Therefore, in order to improve the productivity, a method of suppressing increasing capacity of the reaction system by discharging solvent to outside the system while performing crystallization in a batch operation is being studied. For example, JPH 08-119636 (A) discloses a method of generating nickel hydroxide particles by continuously supplying a nickel salt aqueous solution, ammonia water, and a alkali hydroxide aqueous solution at the fixed ratio into a reaction vessel, removing medium solution so as to mix the reaction liquid before the reaction system overflows from the reaction vessel, and then repeatedly increasing the amount of the reaction system by supplying the reaction liquid and decreasing the amount by removing the medium solution. However, in this method, because increasing the amount of the reaction system by supplying the reaction liquid and decreasing the amount by removing the medium solution is performed repeatedly, there is a problem in that the number of particles per volume of solvent fluctuates, it becomes easy for particle growth to become unstable, and the particle size distribution becomes bad due to fine particles that are newly generated during the reaction.
Moreover, JPH 07-165428 (A) discloses a method of simultaneously and continuously supplying a nickel salt aqueous solution, ammonia water and alkali hydroxide aqueous solution at the fixed ratio to a reaction vessel having a filtering function, and after the amount of the reaction system has reached a specified amount, continuously removing the medium solution from the reaction vessel by using the filtering function, and causing the reaction to occur under a mixing condition while keeping the amount of the reaction system nearly constant. With this method, medium solution is continuously removed, so the number of particles per volume of solvent becomes constant, and particle growth is stable. However, according to the disclosed example, the ratio of the amount of raw material that is supplied after removal of the medium solution has started with respect to the amount of raw material that is supplied up until removal of the medium solution is started is large, and the rate of growth from the particle size of particles that are generated in the initial stage of crystallization to the particle size of the particles in the final stage of crystallization becomes large, so there is a possibility that fine powder will be generated in the reaction solution. Moreover, supposing that the number of particles does not fluctuate and the density remains constant, then, for example, in order to grow 2 μm particles to 12 μm particles, it is necessary to increase the volume by a factor of 216. Therefore, in order to the keep the concentration of the slurry to which raw material has been added in a stirrable state until the particles grow to 12 μm, the initial slurry concentration must be made low. Due to this, a problem occurs in that the time for supplying raw material and causing the raw material to react becomes long, and the productivity is decreased. From the above, it can be easily imagined that by using the method disclosed in JPH 07-165428 (A), producing particles having a large particle size is extremely difficult.
On the other hand, JPH 10-025117 (A) and JP 2010-536697 (A) disclose a method of discharging not only solvent but also particles to outside the reaction system in order to keep the slurry concentration inside the reaction system constant. In these methods, particles that were discharged to outside the reaction system are returned to the reaction system, however, if the particles are not returned to the reaction system, the number of particles in the reaction system decreases, so by causing particle growth in this state it is possible to increase the particle size. However, as in the case of the continuous crystallization operation by overflow, fine particles that have not grown sufficiently, or rough particles that have grown too much are mixed in, so it is difficult to obtain particles having a sharp particle size distribution.
Moreover, in all of the Cited Literature above, the point of maintaining sufficient contact between the cathode active material that is obtained from the nickel composite hydroxide and the electrolyte has not been examined.
As was described above, a method for producing nickel composite hydroxide in an industrial production process such that the nickel composite hydroxide has particles of a suitable size, the particle size distribution is narrow and sufficient contact with the electrolyte can be maintained has not yet been established.